Advanced passive interference management in directional drilling system, apparatus and methods

ABSTRACT

A transmitter for inground use controls a depth signal transmit power in relation to a data signal transmit power such that one reception range of the depth signal at least approximately matches another, different reception range of the data signal. A portable device can form a system with the transmitter in which the portable device scans a plurality of frequencies within at least one low frequency depth signal range to measure the electromagnetic noise at each one of the plurality of frequencies and identify at least one of the frequencies as a potential depth frequency for the transmitter. The portable device can include a dual mode filter having a rebar mode and a normal mode filter. The depth signal frequency is dynamically positionable in relation to low frequency noise.

RELATED APPLICATIONS

This application is a continuation application of copending U.S. patentapplication Ser. No. 17/322,530 filed on May 17, 2021, which is acontinuation application of copending U.S. patent application Ser. No.16/782,948 filed on Feb. 5, 2020 and issued as U.S. Pat. No. 11,008,856on May 18, 2021, which is a divisional application of U.S. patentapplication Ser. No. 16/538,038 filed on Aug. 12, 2019 and issued asU.S. Pat. No. 10,598,007 on Mar. 24, 2020, which is a divisionalapplication of U.S. patent application Ser. No. 15/635,884 filed on Jun.28, 2017 and issued as U.S. Pat. No. 10,378,338 on Aug. 13, 2019, thedisclosures of which are hereby incorporated by reference in theirentirety.

BACKGROUND

The present application is generally related to the field ofcommunications relating to an inground device and, more particularly, toadvanced passive interference management in a directional drillingsystem, apparatus and associated methods.

A technique that is often referred to as horizontal directional drilling(HDD) can be used for purposes of installing a utility without the needto dig a trench. A typical utility installation involves the use of adrill rig having a drill string that supports a boring tool at a distalor inground end of the drill string. The drill rig forces the boringtool through the ground by applying a thrust force to the drill string.The boring tool is steered during the extension of the drill string toform a pilot bore. Upon completion of the pilot bore, the distal end ofthe drill string is attached to a pullback apparatus which is, in turn,attached to a leading end of the utility. The pullback apparatus andutility are then pulled through the pilot bore via retraction of thedrill string to complete the installation. In some cases, the pullbackapparatus can comprise a back reaming tool which serves to expand thediameter of the pilot bore ahead of the utility so that the installedutility can be of a greater diameter than the original diameter of thepilot bore.

Steering of a boring tool can be accomplished in a well-known manner byorienting an asymmetric face of the boring tool for deflection in adesired direction in the ground responsive to forward movement. In orderto control this steering, it is desirable to monitor the orientation ofthe boring tool based on sensor readings obtained by sensors that formpart of an electronics package that is supported by the boring tool. Thesensor readings, for example, can be modulated onto a locating signalthat is transmitted by the electronics package for reception aboveground by a portable locator or other suitable above ground device. Insome systems, the electronics package can couple a carrier signalmodulated by the sensor readings onto the drill string to then transmitthe signal to the drill rig by using the drill string as an electricalconductor. Irrespective of the manner of transmission of the sensor dataand for a given amount of transmission power, there is a limitedtransmission range at which the sensor data can be recovered withsufficient accuracy. The transmission range can be further limited byactive interference and passive interference. Active interferencegenerally consists of sources of electromagnetic signals present in theoperational region that can overwhelm the signal being transmitted bythe system. Conversely, passive interference serves to block or distortthe transmitted signal, which can lead to reduced range or, in somecases, inaccurate readings. One common source of passive interference isrebar. In addressing the deficiencies of the prior art, Applicants filedcommonly owned U.S. patent application Ser. No. 14/845,231 (hereinafterthe '231 Application), entitled COMMUNICATION PROTOCOL IN DIRECTIONALDRILLING SYSTEM, APPARATUS AND METHOD UTILIZING MULTI-BIT DATA SYMBOLTRANSMISSION, which is hereby incorporated by reference in its entirety.The '231 Application is submitted to provide sweeping benefits over thethen-existing state-of-the-art and continues to provide suchimprovements, however, the present Application brings to light stillfurther advances and improvements particularly with respect to passiveinterference, as will be discussed in detail at appropriate pointshereinafter.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

In one aspect of the disclosure, a transmitter and associated method aredescribed for use in conjunction with a horizontal directional drillingsystem that includes a drill string that extends from a drill rig to aninground tool which supports the transmitter such that extension andretraction of the drill string moves the inground tool through theground during an inground operation. The transmitter includes an antennaand one or more sensors for generating sensor data. An antenna driverelectrically drives the antenna to emit a depth signal responsive to adepth drive input for aboveground reception for use in determining adepth of the inground tool and for electrically driving the antennaresponsive to a data drive input to emit at least one data signalcharacterizing the sensor data using at least one data signal frequency,that is higher in frequency than the depth signal, for abovegroundrecovery of the sensor data. A processor is configured for generatingthe depth drive input at the depth signal frequency and for generatingthe data drive input, characterizing the sensor data, in a way thatcontrols a depth signal transmit power in relation to a data signaltransmit power such that one range of the depth signal at leastapproximately matches another, different reception range of the datasignal.

In another aspect of the disclosure, an antenna driver electricallydrives the antenna of the transmitter to emit a depth signal responsiveto a depth drive signal for aboveground reception to determine a depthof the inground tool and electrically drives the antenna, to emit a datasignal characterizing the sensor data using at least one data signalfrequency, that is higher in frequency than the depth signal, foraboveground recovery of the sensor data. A processor is configured forgenerating the depth drive signal at a depth signal frequency and togenerate the data drive signal at the data signal frequency to controlone reception range of the depth signal in relation to another,different reception range of the data signal.

In still another aspect of the disclosure, a system, portable device andan associated method are described in which a transmitter is configuredto move through the ground in a region during an operational procedurewhile transmitting a depth signal at a depth frequency that isselectable in a depth signal frequency range below 1 KHz to provide atleast some immunity to passive interference and which is also subject toelectromagnetic noise that can vary within the region. A portable deviceis configured to scan a plurality of frequencies within the depth signalfrequency range to measure the electromagnetic noise at each one of theplurality of frequencies and identify a lowest one of the frequencies asa potential depth frequency which satisfies a low noise requirementbased on the measured noise.

In a continuing aspect of the disclosure, a portable device serves aspart of a system in which a transmitter is configured to move throughthe ground in a region during an operational procedure whiletransmitting a depth signal at a depth frequency. The portable deviceincludes an antenna for receiving the depth signal to produce an output.A receiver is configured to measure the electromagnetic noise based onthe antenna output in at least two different frequency bands below 1 KHzby incrementally scanning each frequency band to generate a plurality ofincremental noise readings across each frequency band and display one ormore potential depth frequencies for each frequency band based on theincremental noise readings for selection by an operator of one of thepotential depth frequencies as the depth frequency for the depth signal.

In another aspect of the disclosure, a system and associated method aredescribed in which a transmitter is configured to move through theground in a region during an operational procedure while transmitting adepth signal at a depth frequency that is selectable in a depth signalfrequency range below 1 KHz to provide at least some immunity to passiveinterference and which is also subject to electromagnetic noise below 1KHz that can vary within the region. A portable device is configured tomeasure the electromagnetic noise in at least two different frequencybands below 1 KHz by incrementally scanning each frequency band togenerate a plurality of incremental noise readings across each frequencyband and display one or more potential depth frequencies for eachfrequency band based on the incremental noise readings for selection byan operator of one of the potential depth frequencies as the depthfrequency.

In a further aspect of the present disclosure, a portable device andassociated method are described as part of a system in which atransmitter is configured to move through the ground in a region duringan operational procedure while transmitting a depth signal at a depthfrequency and a data stream at one or more data frequencies that atleast characterizes an orientation parameter of the transmitter. Theportable device includes an antenna for receiving the depth signal andthe data frequencies to produce an output. A switchable filter sectionlimits the antenna output in a normal mode to one frequency band that isabove a predetermined frequency to pass the depth signal and the datastream at one or more frequencies and for limiting the antenna output ina rebar mode to another frequency band to pass the depth signal at lessthan the predetermined frequency and the data stream above thepredetermined frequency. A processor is configured for switching theswitchable filter section between the normal mode and the rebar mode torecover the depth signal and the data stream responsive to selection ofthe normal mode and the rebar mode.

In another aspect of the disclosure, a system, method and apparatus aredescribed in which a transmitter is configured to move through theground in a region during an operational procedure while transmitting adepth signal at a selectable depth signal frequency that is settable toany one of a plurality of incrementally spaced apart frequencies in adepth signal frequency range below 1 KHz to provide at least someimmunity to passive interference and which is also subject to lowfrequency electromagnetic noise below 1 KHz or that can vary within theregion and to transmit a data signal that at least characterizes anorientation of the transmitter in a data frequency range above 1 KHz. Aportable device is configured to scan the plurality of incrementallyspaced apart frequencies below 1 KHz to measure the electromagneticnoise at each one of the plurality of incrementally spaced apartfrequencies to identify at least one low noise frequency for setting thedepth frequency in the transmitter to dynamically position the depthsignal frequency in relation to the low frequency noise.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Example embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be illustrative rather than limiting.

FIG. 1 is a diagrammatic view of an embodiment of a system forperforming an inground operation in accordance with the presentdisclosure.

FIG. 2 is a diagrammatic, partially cutaway view, in perspective, whichillustrates an embodiment of a transmitter produced in accordance withthe present disclosure.

FIG. 3 is a block diagram illustrating additional details of thetransmitter with respect to the embodiment of FIG. 2.

FIG. 4 is a block diagram illustrating details of an embodiment of afrequency synthesizer which forms part of the embodiment of thetransmitter of FIGS. 2 and 3.

FIG. 5a is a diagrammatic representation of a lookup table that can beused for the depth and symbol frequency lookup tables shown in FIG. 4.

FIGS. 5b is a diagrammatic representation of embodiments of antennadrive waveforms based on increasing frequency.

FIG. 5c is a diagrammatic representation of one embodiment oftransmitter bands for use in a rebar mode.

FIG. 6a is a plot of the power spectral density of noise taken at a highresolution, corresponding to an actual physical location at which a 50Hz powerline frequency is in use.

FIG. 6b is a diagrammatic illustration of one embodiment of a screenshot showing a display 36 including a bar graph illustrating the averagenoise per frequency sub-band for the embodiment of transmitter bandsshown in FIG. 5 b.

FIG. 6c is a diagrammatic illustration of another embodiment of a screenshot showing display 36 including a bar graph display illustratingmeasured noise for a noise scan in a rebar mode for the embodiment oftransmitter bands shown in FIG. 5 c.

FIG. 7 is a further enlarged view of sub-band 10 from FIG. 6a , shownhere to facilitate a discussion of the section of a depth frequency andsymbol frequencies.

FIG. 8 is a flow diagram that illustrates an embodiment for theoperation of a transmitter according to the present disclosure.

FIG. 9a is a block diagram illustrating an embodiment of the portabledevice shown in FIG. 1.

FIG. 9b is a plot that diagrammatically illustrates an embodiment of anormalized filter response of a normal mode filter shown in FIG. 9 a.

FIG. 9c is a plot that diagrammatically illustrates an embodiment of anormalized response of a rebar mode filter shown in FIG. 9 a.

FIG. 10a is an expanded view of frequency sub-band 6 from FIG. 6 a.

FIG. 10b is a diagrammatic illustration of one embodiment of a screenshot illustrating the appearance of sub-band 6 on a display screen forpurposes of operator selection and modification of symbol frequencies,as well as other functions in accordance with the present disclosure.

FIG. 11 is a further enlarged view of a portion of sub-band 6 of FIGS.10a and 10b , shown here for purposes of describing further details withrespect to symbol frequency selection.

FIG. 12a is a flow diagram illustrating an embodiment of a method foroperating a portable device in accordance with the present disclosurefor purposes of spectral scanning and symbol frequency assignment foroperation in a normal mode.

FIG. 12b is a flow diagram illustrating another embodiment of a methodfor operating a portable device in accordance with the presentdisclosure for purposes of spectral scanning and frequency selection foroperation in a rebar mode.

FIG. 12c is a flow diagram illustrating yet another embodiment of amethod for operating a portable device in accordance with the presentdisclosure for purposes of spectral scanning and frequency selection foroperation in the rebar mode.

FIG. 12d is a flow diagram illustrating still another embodiment of amethod for operating a portable device in accordance with the presentdisclosure for purposes of spectral scanning and frequency selection foroperation in the rebar mode.

FIG. 12e is a flow diagram illustrating yet another embodiment of amethod for operating a portable device in accordance with the presentdisclosure for purposes of spectral scanning and symbol frequency foroperation in the rebar mode.

FIG. 13 is a flow diagram illustrating an embodiment of a method foroperating a portable device in accordance with the present disclosurefor receiving a depth signal and a data signal during an ingroundoperation.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments will be readily apparent to those skilled inthe art and the generic principles taught herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown, but is to be accorded the widest scopeconsistent with the principles and features described herein includingmodifications and equivalents. It is noted that the drawings are not toscale and are diagrammatic in nature in a way that is thought to bestillustrate features of interest. Descriptive terminology may be adoptedfor purposes of enhancing the reader's understanding, with respect tothe various views provided in the figures, and is in no way intended asbeing limiting.

By way of introduction, Applicants recognize that, while the prior arthas attempted to address the problem of passive interference, forexample, caused by the presence of rebar, there remains a need forimprovement. Applicants submit that passive interference is a persistentproblem that represents one of the most challenging concerns that mustbe overcome on a jobsite in addition to active interference and, untilnow, this concern has been unresolved to an acceptable degree by theprior art. Applicants now recognize that the reception of anelectromagnetic signal is impacted in one way by passive interferencefor purposes of determining the depth and location of an ingroundtransmitter and impacted in an entirely different way for purposes ofrecovering a data stream from an electromagnetic signal thatcharacterizes the orientation and other parameters of an ingroundtransmitter. In this regard, determination of the depth and location ofthe inground transmitter depends upon the shape and signal strength ofthe electromagnetic field or flux lines of the electromagnetic signal.The field shape of the electromagnetic signal, however, can be distortedby electrical conductors such as, for example, rebar. This distortioncan lead to inaccurate depth and location determination. While thedistortion can be reduced by decreasing the frequency of theelectromagnetic signal, Applicants recognize that the subject distortionbears little, if any, impact on recovering a data stream or data from anelectromagnetic signal that characterizes the orientation of theinground transmitter. That is, data can be decoded from a higherfrequency electromagnetic signal, irrespective of field distortion, solong as there is sufficient signal strength for purposes of decoding thereceived signal. Accordingly, Applicants bring to light the separationof what is considered to be an ultralow depth frequency (e.g., at orbelow 1 KHz or at or below 1.5 KHz) from one or more data signalfrequencies such that the depth frequency exhibits higher immunity frompassive interference than a higher data signal frequency above anysuitable frequency threshold or limit such as, for example, above 1 KHzor 1.5 KHz, which is more impacted by passive interference than thedepth signal frequency in terms of field shape, but nevertheless servesto transmit the data stream despite the additional field distortion inthe presence of passive interference. At the same time, such lowfrequency ranges can tend to be ultra noisy. Applicants resolve thisconcern, based on a further capability to scan the electromagnetic noisein an appropriate low frequency band such as, for example, from apowerline frequency up to suitable upper frequency limit 1 KHz or 1.5KHz to determine one or more frequencies in the low frequency band thatare at free of or at least relatively low in noise, thereby avoiding thepresence of excess noise. Applicants believe that the combined featuresdisclosed above and elsewhere herein have not been seen in the prior artat least for the reason that a primary perception in the prior art wasthat ultralow frequencies (e.g., at or below 1 KHz or 1.5 KHz) are notpractical depth frequencies based on the presence of high activeinterference at such low frequencies. That is, one of ordinary skill inthe art would dismiss the idea of using such a low frequency out-of-handas likely rendering a system as incapable of receiving the depth signal,and potentially even the data signal, due to the need to admitadditional low frequency active interference in order to receive the lowfrequency depth signal. It is submitted that the present applicationsweeps aside the concerns of the prior art. A bit, for purposes of thepresent application, is a binary data value having two statescharacterized such as 1/0, ±, and the like. A symbol, for purposes ofthe present disclosure, is a data value that represents one or morebits. A multi-bit symbol represents two or more bits. A symbol cancharacterize any suitable type of information such as, for example,pitch data, roll data, temperature data, battery data andsynchronization data, without limitation. Different multi-bit symbolsrepresent different, multi-bit data values. For example, 16 differentsymbols can represent a four bit data value. Each multi-bit symbol, forpurposes of the present disclosure, is represented by a distinctfrequency that is different from the frequency that is associated withany other multi-bit symbol. A symbol stream is made up of a serialtransmission of multi-bit symbols such that the symbol stream isdecodable into a corresponding digital data stream, which can be binary.The symbol stream can be transmitted subject to a packet structure suchthat the particular position of a given symbol within the packetstructure defines a data type that is associated with that symbol.

Turning now to the drawings, wherein like items may be indicated by likereference numbers throughout the various figures, attention isimmediately directed to FIG. 1, which illustrates one embodiment of asystem for performing an inground operation, generally indicated by thereference number 10. The system includes a portable device 20 that isshown being held by an operator above a surface 22 of the ground as wellas in a further enlarged inset view. It is noted that only limitedinter-component cabling is shown within device 20 in order to maintainillustrative clarity, but all necessary cabling is understood to bepresent and may readily be implemented by one having ordinary skill inthe art in view of this overall disclosure. Device 20 includes athree-axis antenna cluster 26 measuring three orthogonally arrangedcomponents of magnetic flux. One embodiment of a useful antenna clustercontemplated for use herein is disclosed by U.S. Pat. No. 6,005,532which is commonly owned with the present application and is incorporatedherein by reference. Details with respect to the embodiment of theantenna utilized herein will be provided at an appropriate pointhereinafter. Antenna cluster 26 is electrically connected to anelectronics section 32. A tilt sensor arrangement 34 can be provided formeasuring gravitational angles from which the components of flux in alevel coordinate system may be determined. An appropriate tilt sensorincludes, by way of non-limiting example, a tri-axial accelerometer.

Device 20 can further include a graphics display 36 and a telemetryantenna 40. The latter can transmit or receive a telemetry signal 44 fordata communication with the drill rig. It should be appreciated thatgraphics display 36 can be a touch screen in order to facilitateoperator selection of various buttons that are defined on the screenand/or scrolling can be facilitated between various buttons that aredefined on the screen to provide for operator selection. Such a touchscreen can be used alone or in combination with an input device 48 suchas, for example, a trigger button. The latter can be used without theneed for a touch screen. Moreover, many variations of the input devicemay be employed and can use scroll wheels and other suitable forms ofselection device either currently available or yet to be developed. Theelectronics section can include components such as, for example, one ormore processors, memory of any appropriate type, antenna drivers andanalog to digital converters. As is well known in the art, the lattershould be capable of detecting a frequency that is at least twice thefrequency of the highest frequency of interest. Other components may beadded as desired such as, for example, a magnetometer 50 to aid inposition determination relative to the drill direction and ultrasonictransducers for measuring the height of the device above the surface ofthe ground.

Still referring to FIG. 1, system 10 further includes drill rig 80having a carriage 82 received for movement along the length of anopposing pair of rails 84. An inground tool 90 is attached at anopposing end of a drill string 92. By way of non-limiting example, aboring tool is shown as the inground tool and is used as a framework forthe present descriptions, however, it is to be understood that anysuitable inground device may be used such as, for example, a reamingtool for use during a pullback operation or a mapping tool. Generally,drill string 92 is made up of a plurality of removably attachable drillpipe sections such that the drill rig can force the drill string intothe ground using movement in the direction of an arrow 94 and retractthe drill string responsive to an opposite movement. The drill pipesections can define a through passage for purposes of carrying adrilling mud or fluid that is emitted from the boring tool underpressure to assist in cutting through the ground as well as cooling thedrill head. Generally, the drilling mud also serves to suspend and carryout cuttings to the surface along the exterior length of the drillstring. Steering can be accomplished in a well-known manner by orientingan asymmetric face 96 of the boring tool for deflection in a desireddirection in the ground responsive to forward, push movement which canbe referred to as a “push mode.” Rotation or spinning 98 of the drillstring by the drill rig will generally result in forward or straightadvance of the boring tool which can be referred to as a “spin” or“advance” mode.

The drilling operation can be controlled by an operator (not shown) at acontrol console 100 which itself includes a telemetry transceiver 102connected with a telemetry antenna 104, a display screen 106, an inputdevice such as a keyboard 110, a processing arrangement 112 which caninclude suitable interfaces and memory as well as one or moreprocessors. A plurality of control levers 114, for example, controlmovement of carriage 82. Telemetry transceiver 104 can transmit orreceive a telemetry signal 116 to facilitate bidirectional communicationwith portable device 20. In an embodiment, screen 106 can be a touchscreen such that keyboard 110 may be optional.

In an embodiment, device 20 is configured for receiving anelectromagnetic depth signal 120 and an electromagnetic data signal 122that are transmitted from a transmitter 130 that is supported within theboring tool or other inground tool. These signals may be referred tocollectively herein as the transmitter signals. The transmitter signalscan be dipole signals. It should be appreciated that the portable devicecan be operated in either a walkover locating mode, as illustrated byFIG. 1, or in a homing mode having the portable device placed on theground, for example, as illustrated by commonly owned U.S. Pat. No.9,540,879 which is incorporated by reference in its entirety. While thepresent disclosure illustrates a dipole locating field transmitted fromthe boring tool and rotated about the axis of symmetry of the field, thepresent disclosure is not intended as being limiting in that regard.

Information carried by the data signal can include, but is not limitedto position orientation parameters based on pitch and roll orientationsensor readings, temperature values, pressure values, battery status,tension readings in the context of a pullback operation, and the like.Device 20 receives the transmitter signals using antenna array 26 andprocesses received data signal 122 to recover the data, as will befurther described.

FIG. 2 is a diagrammatic, partially cutaway view, in perspective, whichillustrates an embodiment of transmitter 130. The latter includes a mainhousing 134 that can be at least generally cylindrical in configuration.A battery compartment 138 can be formed at one end of the housing withan opposing end 140 supporting a main printed circuit board (PCB) 144which itself can support an antenna 148 that emits the transmittersignals. An accelerometer module 150 can be positioned adjacent to oneend of PCB 144. Other sensors and components can be located on the mainprinted circuit board, as will be further described.

Attention is now directed to the block diagram of FIG. 3 in conjunctionwith FIG. 2 for purposes of describing additional details with respectto an embodiment of transmitter 130. The transmitter includes aprocessing section 152 that receives sensor information via amultiplexer 154. The multiplexer can be interfaced with any number ofsensors forming a sensor suite. In the present example, the sensorsinclude accelerometers 158 that are supported in accelerometer module150 of FIG. 2, a pressure sensor 160 which can be used to sense theannular pressure within the borehole around the transmitter, atemperature sensor 164, a battery current sensor 168 and a batteryvoltage sensor 170. External communication for the transmitter can beprovided, in some embodiments, by an external communication connection174. Such communication is not required to be transmitted through theground but rather can be performed while the transmitter is aboveground, for example, in a position adjacent to device 20. The externalcommunication can be implemented in any suitable manner including butnot limited to IrDA, NFC, Wi-Fi, Zigbee or Bluetooth. A power supplysection 178 can comprise a battery 180 that provides power via anovervoltage and reverse polarity detector 184. The latter provideselectrical power to a logic and sensor power supply 188 and to anantenna drive power supply 190. The logic and sensor power supplyprovides power to the sensor suite as well as to processing section 152.The antenna drive power supply feeds electrical power to a depth antennadriver 194 and a data antenna driver 198 which electrically driveopposing ends of an antenna coil forming part of antenna 148. Drivers194 and 198, in an embodiment, can be half bridge drivers. The antennadrivers receive input signals from a processor 200 that forms part ofthe processing section. The processing section further includes anoscillator 210 such as, for example, a crystal oscillator. Theoscillator can be selected to provide a relatively high degree oftemperature and overall stability. Processor (CPU) 200 includes a timersection 212 that can serve to generate a reference signal having astability that reflects the stability of oscillator 210. The outputfrequency of the timer is selectable based on a reload timer value thatcan be specified by the user. The processor is in data communicationwith a memory 218 which can include any suitable information including,but not limited to depth frequency information 224 and symbol frequencyinformation 228, each of which will be described at an appropriate pointhereinafter.

Turning to FIG. 4, an embodiment of a frequency synthesizer is generallyindicated by the reference number 300 and is implemented as part ofprocessing section 152 of FIG. 3. It should be appreciated that thefrequency synthesizer can be implemented in hardware, software or anysuitable combination thereof. The frequency synthesizer can be anysuitable embodiment either currently available or yet to be developed.The embodiment of FIG. 4 is a two channel direct digital synthesizer(DDS) having a depth channel 304 and a symbol channel 308. The depthchannel provides an output signal 310 to depth driver 194 of FIG. 3 forproducing depth signal 120 while the symbol channel provides an outputsignal 312 to data driver 198 of FIG. 3 for producing data signal 122(FIGS. 1 and 2). A depth channel waveform lookup table section 320 and asymbol channel waveform lookup table section 324 each includes at leastone waveform or phase lookup table that characterizes one period of aselected waveform such as, for example, a sinusoid. In anotherembodiment, each of the depth channel lookup table section and thesymbol channel lookup table section can include a plurality of waveformor phase lookup tables. In the present example, there is one waveformlookup table diagrammatically shown and indicated by the referencenumber 326 in each of the depth channel and symbol channel lookup tablesections. It should be appreciated that any desired waveform orwaveforms can be characterized by the lookup table(s). Further, there isno requirement for the depth channel lookup table(s) and the symbolchannel lookup table(s) to characterize the same waveform(s). In someembodiments of a frequency synthesizer, there is no requirement for alookup table. For example, a suitable mathematical expression can beused.

FIG. 5a is a graphical illustration of lookup table 326 which caninclude a large number of samples of the magnitude of the characterizedwaveform based, for example, on the amount of memory that is availableand the desired resolution. Given that the depth channel and the symbolchannel use the same lookup table in the present embodiment, it shouldbe appreciated that it is only necessary to store a single copy foraccess by both channels. In the present embodiment, lookup table 326represents one period of a sinusoidal waveform. The vertical axisrepresents Pulse Width Modulation (PWM) percentage with the positivewaveform peak at 100 percent and the negative waveform peak at 0 percentfor reasons yet to be described. The horizontal axis of the plotrepresents time slots such that a given time slot has an associatedamplitude. The time slot values can be referred to as samples that areselectively addressable by a depth channel phase accumulator 330 and asymbol channel phase accumulator 334, respectively, using an m-wideaddressing arrangement. It is noted that a very large number of samplescan be associated with the lookup table. Each phase accumulator isconfigured to provide an output count to the lookup table section basedon an input increment or offset size that is provided by a depth channelfrequency control 338 and a symbol channel frequency control 340,respectively. Each phase accumulator generates what can be described asa quantized sawtooth waveform output that changes from one level orcount to the next by a respective one of the input increment sizes. Inresponse to each respective phase accumulator input count, for eitherthe depth channel or the symbol channel, lookup table 326 sequentiallygenerates digital output magnitudes that are received by a depth channelpulse width modulator (PWM) generator 350 and a symbol channel pulsewidth modulator (PWM) generator 352, respectively, on an n-wide addressarrangement. Based on the magnitude value received by each PWMgenerator, a pulse width modulator generates an output pulse trainhaving an at least generally constant output magnitude but with a pulsewidth that increases in proportion to the output magnitude value fromthe lookup table. Filtering, via the inductive properties of antenna148, smooths the waveform to approximate a desired output waveform suchas, for example, a sinusoidal waveform.

Referring again to FIG. 4, each of a depth channel output waveform 360and a symbol channel output waveform 362 can be generated, for example,across a frequency range approaching 0 Hz to 45 KHz with a high degreeof accuracy. It should be appreciated that any suitable frequency rangecan be utilized and the range of 0 to 45 KHz has been described by wayof example and is not intended to be limiting. In the presentembodiment, the accuracy can be at least approximately ±0.1 Hz or lessat a resolution of at least approximately 5 Hz. It is noted that thespecified accuracy, in the context of the present embodiment, is givenfor at least approximately 45 KHz which represents a lower limit onaccuracy across the frequency range. As compared to prior artapproaches, it should be appreciated that the present disclosureprovides for higher precision, greater consistency and remarkableflexibility with respect to frequency placement across the entiretransmission bandwidth. Output frequencies 360 and 362 are establishedbased on the input increment size provided to depth channel phaseaccumulator 330 via depth channel frequency control 338 and symbolchannel phase accumulator 334 via symbol channel frequency control 340.Depth channel frequency control 338 receives a depth frequency input 368that specifies the depth frequency. The depth channel frequency controlcan convert a specified depth frequency to an increment size for depthchannel phase accumulator 330 in any suitable manner. In an embodiment,the depth channel frequency control can include an increment lookuptable 370 that indexes depth frequency against the increment size. Inanother embodiment, a formula can be used to determine the incrementsize, as follows:

$\begin{matrix}{{{increment}{size}} = \frac{\left( {{desired}{frequency}} \right) \times \left( {{phase}{accumulator}{size}} \right)}{\left( {{phase}{accumulator}{update}{rate}} \right)}} & \left( {{EQN}1} \right)\end{matrix}$

Where the phase accumulator size is chosen to provide the minimumrequired frequency resolution and the phase accumulator update rate isestablished by timer 212 (FIG. 3). Similarly, the symbol channelfrequency control can convert a specified symbol frequency received on adata symbol stream input 374 to an increment size for symbol channelphase accumulator 334 in any suitable manner such as, for example, byusing increment lookup table 370 or a formula. The origin of the datasymbol stream for data symbol stream input 374 will be described at anappropriate point hereinafter. It is noted that there is no requirementfor the depth and symbol channel frequency controllers to use anidentical increment size lookup table. Table 1 below illustrates aportion of an embodiment of increment lookup table 370.

TABLE 1 Desired Output Frequency vs. Phase Accumulator Size IncrementDesired Output Frequency (Hz) Phase Accumulator Increment (counts) 5 150 10 500 100 32770 6554 45000 9000

Based on Table 1, it should be appreciated that a high degree ofresolution is provided in terms of the frequency that is selectable foreach of depth output frequency 360 and symbol output frequency 362. Inthe present embodiment, a resolution of 5 Hz can be provided across theentire frequency range extending from worldwide AC powerline frequenciesto 45 KHz. Of course, other embodiments can utilize a like or differentresolution to even higher frequencies. Other resolutions can be used,some of which are larger and some of which are even more fine, however,Applicants recognize that 5 Hz represents a relatively small commonmultiple of 50 Hz and 60 Hz which are the predominant powerlinefrequencies around the world. Further discussions with respect topowerline frequencies will be presented below.

With continuing reference to FIG. 4, it should be appreciated that depthoutput frequency 360 and symbol output frequency 362 are illustrated asfrequency tones that are of a limited or fixed duration, an at leastessentially fixed frequency and can include a variable magnitude. Inthis regard, primary amplitude control can be provided based on amultiplier that can be specified by a multiplier table 376 that isaccessible by both depth channel frequency control 338 and symbolchannel frequency control 340. The multiplier is specified in the rangefrom 0 to 1.0. In order to produce a desired transmission power for agiven frequency, samples obtained from lookup table 326 are multipliedby the multiplier. Thus, a multiplier of 1.0 produces maximum or 100percent amplitude whereas a multiplier of zero produces an output ofzero. As will be further described, the multiplier table can be used tocompensate for changes in coupling between the antenna and drivecircuits as well as changes in antenna impedance responsive to varyingfrequency.

Again, only one copy of the multiplier table need be stored if the sametable is used by both channels. Further, magnitude/amplitude shaping canbe accomplished using a depth channel waveform/amplitude control 380 forthe depth channel which may be referred to as a depth channel shaper anda symbol channel waveform/amplitude control 382 which may be referred toas a symbol channel shaper. Another example output of depth channel PWMgenerator 350 is a continuous depth signal 386 which is of at leastessentially a continuous magnitude. In this instance, depth channelshaper may not be needed, although it should be understood that itsoperation reflects the operation of the symbol channel shaper, asdescribed herein. It should be appreciated that the depth of thetransmitter, based on depth signal 386, can be determined based on thewell-known dipole equations, as described for example, in U.S. Pat. No.5,633,589 which is incorporated herein by reference. Another exampleoutput 390 of symbol channel PWM generator 352 illustrates a series ofoutput symbols indicated as 392 a-392 f which can vary in frequency fromone symbol to the next. As will be further described, output 390 cancomprise a symbol stream. In the present embodiment, there is no gap orzero magnitude space present or inserted between adjacent symbols byphase accumulator 334. Thus, the frequency can change abruptly from onesymbol to the next in a way that can introduce noise responsive to suchabrupt frequency transitions. It should be appreciated that symbols 392a-392 f are shaped in a way that avoids abrupt frequency transitions bybeginning and ending at a value of approximately zero magnitude. Suchshaping can be accomplished through the application of a suitable windowor tapering function by symbol channel shaper 382 such as, for example,a Hamming window, Hann window, Welch window or a triangular window,among others. What is common to all of the subject window functionsresides in a zero magnitude of the waveform for any point that isoutside of a window interval such that each symbol starts and ends witha zero magnitude waveform.

Attention is now directed to FIG. 5b and Table 2 in conjunction withFIGS. 2, 4 and 5 a. Although not a requirement, embodiments oftransmitter 130 can be configured to transmit depth signal 120 and datasignal 122 using a series of transmitter bands, generally indicated bythe reference number 400 that extend from approximately 0 to 45 KHz, asshown in FIG. 5b . It should be understood that other embodiments canuse different transmitter bands and sub-bands with the presentembodiment serving by way of a non-limiting example. While the value ofzero is listed as a lower limit, it should be understood that the actuallower limit can be represented by worldwide predominant power linefrequencies or some higher value. The transmitter bands are indicated asBT1-BT5 and are also set forth in Table 2. While the descriptiveframework employed by FIG. 5b and Table 2 uses transmitter bands thatinclude frequency sub-bands, it should be appreciated that thisband/sub-band nomenclature can be varied in any suitable manner, forexample, so long as there is an appropriate correspondence with thefinal column of Table 2 which comprises multiplier table 376. It isnoted that the value of the multiplier from one sub-band to the next inmultiplier table 376 progressively increases in the illustratedembodiment, which is also reflected by the progressively increasingmagnitudes of sub-band waveforms 402 a-402 j in FIG. 5b . In anotherembodiment, each sub-band waveform 402 a-402 j can be a separate lookuptable to make up a set of lookup tables, instead of using a multipliertable with a single lookup table. In another embodiment, there is noneed to define frequency bands or sub-bands since the multiplier can bespecified, for example, as a function. As noted above, there is norequirement to use a lookup table. For example, in an embodiment, anexpression can be used in the form of a function that is linear and cangive an essentially continuous variation in the multiplier value at thetransmit frequency resolution of the transmitter. Other functions can bedeveloped, for instance, using an appropriate curve fitting approachsuch as, for example, least squares.

TABLE 2 Transmitter Bands and Sub-Bands Transmitter Band Sub-BandSub-Band Multiplier Band Frequency Range No. Frequency Range Table 376BT1       0-4.5 KHz SB1       0 to 4.5 KHz 0.34 BT2 4.5 KHz-9 KHz SB24.5 KHz to 9 KHz 0.45 BT3   9 KHz-18 KHz SB3   9 KHz to 13.5 KHz 0.55SB4 13.5 KHz to 18 KHz 0.63 BT4    18 KHz-31.5 KHz SB5    18 KHz to 22.5KHz 0.70 SB6 22.5 KHz to 27 KHz 0.75 SB7    27 KHz to 31.5 KHz 0.82 BT531.5 KHz-45 KHz SB8 31.5 KHz to 36 KHz 0.86 SB9    36 KHz to 40.5 KHz0.92 SB10 40.5 KHz to 45 KHz 1.00

Still referring to FIG. 5 b, the frequency range from 0 to 45 KHz, inaccordance with the present embodiment, is further divided into 10sub-bands SB1-SB10, each of which is 4.5 KHz in width. Each band aboveBT1 and sub-band 1 can be considered as including its lower frequencylimit. The use of the transmitter bands, although not required, allowsfor managing transmission efficiency as well as transmission power.While transmitter bands BT1 and BT2 each include a single sub-band, itis noted that transmitter band BT3 includes two sub-bands, SB3 and SB4,and transmitter bands BT4 and BT5 each include three sub-bands: SB5-SB7and SB8-SB10, respectively. An embodiment of a transmitter according tothe present disclosure can be configured to transmit depth signal 120and data signal 122 in a single sub-band. In another embodiment, atransmitter can be configured to transmit depth signal 120 in band thatis different from the band that is used for data signal 122. In thisregard, it should be appreciated that the use of a separate synthesizerchannel (FIG. 4) for the depth channel provides for a great degree offlexibility with regard to the frequency of the depth signal in relationto the data signal. In a wideband transmitter, as further describedbelow, the transmitter can transmit on two or more bands such that thebands can even be spaced apart by other bands.

In some embodiments, transmitter 130 can be configured to cooperate withantenna 148 such that the transmitter transmits over a wide frequencyrange or band extending from a lowermost frequency to approximately 45KHz or higher. In this way, this wide frequency band can be covered by asingle wideband transmitter, using a single antenna, while maintainingsuitable efficiency with respect to power consumption across the entirewide frequency range. In order to transmit across an entire frequencyrange from a lowermost frequency to approximately 45 kHz, by way ofnon-limiting example, embodiments of multiplier table 376 can beconfigured to include any suitable number of values. Depth channel phaseaccumulator 330 and symbol channel phase accumulator 334 can beconfigured to utilize the appropriate entry in multiplier table 376based on the frequency to be generated. In this way, an at leastgenerally constant power consumption can be maintained over the entirewide transmission bandwidth, irrespective of transmit frequency. In theprior art, given a constant drive voltage and waveform, the transmitterwould otherwise draw increasingly more power as the frequency isreduced. The antenna, in the embodiments presented herein, is notrequired to be driven at a resonant frequency. The resonant frequencythat is presented by the inductance of antenna 148, in combination withany parasitic capacitances, is generally far higher than a highestfrequency of the transmission range such as, for example, 45 kHz. Forinstance, the resonant frequency can be in the megahertz range.

In one embodiment of a wideband transmitter, the transmitted frequencyset can range from SB3 through SB10, corresponding to a widebandfrequency range of 9 KHz to 45 KHz. There is no need for frequencyconfinement in any of these sub-bands (see, for example, Table 2) in thecontext of a wideband transmitter.

Based on the foregoing, the present disclosure can provide a widebandtransmitter having a single antenna that is driven across a widefrequency band in a way that can maintain constant or controlled powerconsumption, at least to an approximation, when the power consumptionwould otherwise exhibit large variations across that same frequency bandby using a single drive signal waveform or lookup table in conjunctionwith a multiplier value. Variation in the power consumption across thewide frequency band can be limited to acceptably low levels across therange of 9 KHz to 45 kHz. In this way, Applicants are able to provide awideband transmitter that operates across a wide frequency range withpower consumption regulation and control that is submitted to have beenunseen heretofore. In the past, performing inground operations atdifferent frequencies for depth and locating data often required thepurchase of a transmitter that was dedicated to each frequency ofinterest. The recognitions that have been brought to light herein canresult in significant cost savings since a single wideband transmittercan replace a plurality of prior art transmitters. In this regard, theteachings herein are equally applicable with respect to a transmitterthat transmits a depth frequency or tone at one discrete frequency andtransmits a data signal at a different frequency that is modulated inany suitable manner such as, for example, using BPSK, QPSK or Manchesterencoding.

As discussed above, Applicants recognize that there are benefitsassociated with transmitting the depth frequency or tone at a relativelylow or ultralow frequency such as, for example, 1 kHz or lower andtransmitting a signal frequency at one or more higher frequencies suchas, for example, in a range extending upward from 1 KHz or higher, suchas, for example, higher than 4.5 KHz. In an embodiment, transmitter 130includes what may be referred to as a “rebar” or passive interferencemode in order to lessen the effects of passive interference while, atthe same time, preserving data throughput at the data frequencies. Inthis regard, low depth tone frequencies avoid sources of passiveinterference such as rebar at the risk of exposure to additional lowfrequency active interference in the absence of the provisions disclosedherein, while higher data frequencies are associated with higher ratesof data throughput based on the Nyquist rate. Because the presence, forexample, of rebar can distort, weaken and misshape the electromagneticlocating field of the transmitter, these effects can cause difficulty inaccurately locating the position of the inground transmitter, whichrelies on the shape of the locating field, as well as presentingincorrect depth readings which rely on the signal strength of thelocating signal. For example, the point at the surface of the groundthat is indicated to be directly above the transmitter can be shiftedaway from its actual position. As another example, passing under rebarcan produce a sudden increase in the displayed depth of the transmitterof an inground tool responsive to signal attenuation caused by therebar. Using a relatively lower frequency for the depth signal isbeneficial in terms of avoiding these effects. Until now, it issubmitted that transmitting the depth signal at the low frequenciesdisclosed herein such as, for example, below 1 KHz, presented technicalchallenges that were perceived to be insurmountable in the prior artincluding the aforementioned presumption that ultralow depth frequenciesare subject to too much active interference to be practical. The presentdisclosure is submitted to sweep aside the perceived limitations of theprior art with a heretofore unseen combination of features.

The present disclosure allows for the transmission of a depth tone thatis spaced apart from data frequencies by an amount that is submitted tobe heretofore unseen, particularly when a single antenna is used totransmit both. For example, the depth tone can be transmitted at 1 kHzor less, as will be described immediately hereinafter.

Attention is now directed to FIG. 5c which is a diagrammaticillustration of a series of transmitter bands, generally indicated bythe reference number 420 for an embodiment of the rebar mode. It isnoted that all frequency values are given in KHz in the figure. Thepresent embodiment also includes previously described transmitter bandsBT2-BT3 which were set forth in Table 2 and repeated in Table 3. In thisembodiment, a set of rebar bands 424 includes rebar low band, RBL,extending from approximately 330 Hz to 400 Hz; a rebar middle band, RBM,extending from approximately 405 Hz to 575 Hz; and a rebar high band,RBH, extending from approximately 580 Hz to 750 Hz. It is noted that thenumber of rebar bands and their endpoints can be varied in any suitablemanner. For example, RBH can include an upper limit of 1 KHz or someother value between 1 KHz and the lower limit of BT2 at 4.5 KHz such as,for example, 1.5 KHz. As another example, RBL can include a lower limitapproaching a powerline frequency. As still another example, a super lowband can be added below RBL which can have a lower limit approaching apowerline frequency. As yet another example, an additional band can beinserted between RBH and BT2. Again, there is no need to adopt anapproach that specifies the multiplier on a band-by-band basis, so longas the multiplier value is specified for any potential transmitfrequency that can be selected.

Rebar bands 424 are set forth in Table 3. The final column of Table 3comprises a multiplier table 410 which is an embodiment used in place ofaforedescribed multiplier table 376 of Table 2 when operating in a rebarmode. Multiplier tables can vary in complex ways based on variousfactors, as demonstrated by multiplier table 410, which was empiricallydeveloped. FIG. 5c diagrammatically illustrates the sinusoidal lookuptable subject to multiplier table 410 with waveforms 412 a-412 ccorresponding to RBL, RBM and RBH, respectively. Waveforms 414 a-414 ccorrespond to SB2-SB4, respectively.

TABLE 3 Rebar Mode Transmitter Band Sub-Band Sub-Band Multiplier BandFrequency Range No. Frequency Range Table 410 RBL 330 Hz-400 Hz n/a n/a0.34 RBM 405 Hz-575 Hz n/a n/a 0.39 RBH 580 Hz-750 Hz n/a n/a 0.47 BT24.5 KHz-9 KHz  SB2 4.5 KHz-9 KHz 0.45 BT3  9 KHz-18 KHz SB3   9 KHz to13.5 KHz 0.55 SB4 13.5 KHz to 18 KHz 0.63

Table 4 illustrates example frequencies selected based on noise scanningfor operation of the transmitter in the rebar mode. It is noted thatdata frequencies S0-S15 were all selected from SB4, although this is nota requirement. In this regard, these frequencies can be selected atleast in SB2-SB4 of FIG. 5 c. It should be noted that frequencies inSB2-SB4 generally provide good transmission range with relatively highdata throughput or bandwidth. In this regard, one of the sub-bands canbe selected on the basis of average noise per sub-band seen during anoise scan, either manually, automatically or some combination thereof.At the same time, a depth frequency of 345 Hz provides a remarkablelevel of immunity to passive interference such as that, for example,resulting from rebar. Considering multiplier table 410 of Table 3, themultiplier can be set to 1.0 for the highest frequencies or sub-band tobe transmitted by a given embodiment of the transmitter. The remainingentries in the multiplier table can be set, for example, to provide fora constant power consumption based on power allocated between the depthsignal and the data signal frequencies or based on Applicantsrecognitions that have been brought to light below. In an embodiment,the values for the multiplier table in either the normal or rebar modecan be empirically determined, for example, by adjusting transmit powerof the depth signal in relation to the data signal while observingoverall power consumption of the transmitter such that a maximum powerthreshold is not violated. Such an empirical process is likely bestperformed in a region of low active interference. A particularembodiment can also consider the reception range of the depth signal inrelation to that of the data signal, as will be further discussed below.

TABLE 4 EXAMPLE SELECTED FREQUENCIES FOR REBAR MODE DesignationFrequency (Hz) Depth Signal 345 S0 14,740 S1 14,850 S2 15,085 S3 15,210S4 16,500 S5 16,770 S6 16,695 S7 17,105 S8 17,190 S9 17,225 S10 17,240S11 17,335 S12 17,445 S13 17,560 S14 17,680 S15 17,995

For purposes of comparison, Table 5 sets forth frequency selections forthe normal mode wherein the depth signal frequency is above 1 KHz.

TABLE 5 EXAMPLE SELECTED FREQUENCIES FOR NORMAL MODE DesignationFrequency (Hz) Depth Signal 40,675 S0 40,740 S1 40,850 S2 41,085 S341,210 S4 41,500 S5 41,825 S6 42,235 S7 42,400 S8 42,700 S9 42,845 S1043,205 S11 43,420 S12 43,665 S13 43,825 S14 44,360 S15 44,635

The selected frequencies in Table 5 are chosen from sub-band 10.Applicants recognize that the presence of active inference as detectedin a noise scan, that forms the basis of these frequency selections,does not bear on the issue of passive interference. In this regard, anattempt to use the depth signal frequency of Table 5 may prove to beunworkable, necessitating a switch to the rebar mode which uses a farlower frequency for the depth signal such as, for example, 345 Hz, asseen in Table 4. In another embodiment, frequency selections for thedepth frequency and the data symbols in the normal mode can be confinedto the same frequency range as the data symbol selections for the rebarmode (SB-2 through SB-4 in the example of Table 3). It is noted that insuch an embodiment, the reception range for the data frequencies at agiven transmit power can be at least somewhat improved in the normalmode as compared to the data signal reception range in the rebar mode atleast as a result of the admission of relatively less activeinterference in the normal mode based resulting from front endfiltering, as yet to be discussed.

Having described in detail above transmitters and associated componentsaccording to the present disclosure, details with respect totransmission of data signal 122 will now be brought to light. Inparticular, an M(ary) frequency shift keying approach is used such thata plurality of different symbols can be streamed as data signal 122. Inthe rebar mode, data symbol frequencies can be selected in BT2 and/orBT3 to make up data signal 122 while depth frequency 120 can be selectedwithin the set of rebar bands 424 (FIG. 5c ), based on noise scanning asyet to be described. In the normal or rebar mode, the data signal canserve to transmit a multi-bit symbol stream. The ability to transmit amulti-bit symbol stream is facilitated, at least in part, based on theuse of synthesizer 300 of FIG. 4. In particular, a multi-bit data symbolstream can be provided at data symbol stream input 374 to symbol channelfrequency control 340. In this way, data symbols corresponding to a widevariety of distinct frequencies can be specified as part of the datasymbol stream with each different symbol corresponding to a differentfrequency. In an embodiment, the data symbols of the symbol stream cancorrespond to 16 symbols (4 bits), although any suitable number ofsymbols can be used, based on a desired data throughput. FIG. 4illustrates output 390 based on 16 symbols, S0-S15, with S0corresponding to a lowest frequency and each successivelyhigher-numbered symbol corresponding to a relatively higher frequency,although this is not required and the mapping or assignment of symbolsto frequencies can be performed in any suitable manner. Thus, output 390corresponds to an example input symbol stream of S2, S12, S2, S15, S0and S10 at input 374.

Based on the foregoing, Applicants submit that system 10 can provide alevel of active noise immunity and passive interference immunity thathas heretofore been unseen with respect to performing an ingroundoperation such as, for example, horizontal directional drilling andrelated pull-back or back-reaming operations. Related considerations andfurther details will be provided in the context of a discussion ofdevice 20 which receives the depth signal and the data signal and whichalso can assist in the identification of the depth signal frequency andsymbol frequencies to be used by the transmitter. It should beappreciated that the symbol frequency ordering given by Table 4 is notrequired. In Table 5, the depth signal frequency can be positionedbetween symbol frequencies. Based on the use of a separate channel forpurposes of generating the depth signal (FIG. 4), the depth signal canbe positioned in a different sub-band than the symbol frequencies.Further, the symbol frequencies can be reordered or rearranged in anysuitable manner. With regard to constraining frequency selection to asingle sub-band, it should be understood that an embodiment of awideband transmitter can be configured to operate in a manner thatmimics the operation of a transmitter that is constrained to operatebased on sub-bands. For example, the selected frequencies in a widebandtransmitter can be limited or constrained to a single sub-band, eventhough the wideband transmitter is capable of transmission over a widerange of sub-bands.

FIG. 6a is a plot of the power spectral density of noise taken at a highresolution, generally indicated by the reference number 500,corresponding to an actual physical location at which a 50 Hz powerlinefrequency is in use. The signal level is shown on the vertical axis andthe frequency is shown on the horizontal axis. The frequency range of 0to 45 KHz corresponds to the frequency range that is covered by therange of transmitters described in accordance with the presentdisclosure. Transmitter sub-bands SB1-SB10 are also indicated. It is ofinterest to note that sub-band 1 is quite noisy as compared to the noiseseen in most of the higher frequency sub-bands. In this regard, it issubmitted that one of ordinary skill in the art would have been led toreject the idea of using a depth signal having a frequency below 1 KHzor 1.5 KHz for at least this reason, as will be further discussed.Applicants, however, have brought to light sweeping improvements thatprovide for precise positioning of the depth signal at a low activenoise frequency that can be even lower than 1 KHz.

While the spectral scan of FIG. 6a illustrates spectral informationessentially at a single location, it should be appreciated that spectralinformation can be collected in a cumulative manner. For example,spectral scanning can be performed while an operator walks the plannedborepath with device 20 while the device characterizes the noiseenvironment. In this way, the spectral plot of FIG. 6a can be thought ofas representing the noise environment along the entire planned borepathwith subsequent frequency selections being based on the noiseenvironment as characterized for the entire length of the plannedborepath while still utilizing the frequency selection techniques thathave been brought to light herein.

FIG. 6b illustrates one embodiment of a screen shot showing display 36including a bar graph display illustrating the average noise persub-band for operation in the normal mode wherein sub-band SB-10 ishighlighted, for example, using hatching and/or color or in some othersuitable manner to indicate that SB-10 has been automatically selected.In another embodiment, the locator can make an automatic recommendationbased on average noise per sub-band in conjunction with otherstatistical values. Any suitable statistical value(s) can be utilizedincluding, for example, standard deviation, minimum noise and peaknoise. In still another embodiment, more than one sub-band can berecommended, in which case the user can select between the recommendedsub-bands. Recommending multiple sub-bands can be based on a limitedamount of statistical variation between the sub-bands. For example,sub-bands 9 and 10 can both be recommended based on the relativelylimited difference between the two sub-bands, as seen in FIG. 6 b. Asanother example, multiple sub-bands can be recommended, for instance,based on the average noise for a first sub-band being lower than theaverage noise for a second sub-band while the peak noise for the firstsub-band is higher than the peak noise for the second sub-band. In anembodiment wherein more than one sub-band is recommended, the system canbe configured such that the user can select one of such multiplerecommended sub-bands for transmission. In another embodiment, the usercan select multiple recommended sub-bands for transmission. In yetanother embodiment, one or more of such multiple recommended sub-bandscan be automatically selected for transmission. Since the informationpresented in FIG. 6a is based on a high resolution noise scan using a 5Hz increment, a significant amount of noise information can be extractedfrom the data. For example, the standard deviation of the noise valueswithin each sub-band can be determined. The heights of the various barsin FIG. 6b can be weighted by adding or subtracting a value based on oneor more other statistics. For example, if the standard deviation for agiven sub-band is high, meaning that the noise values are relativelymore widely spread out, the height of the associated bar can bemaintained or even increased by some amount. On the other hand, if thestandard deviation for a given sub-band is low, meaning that the noisevalues within that sub-band are relatively consistent, the height of theassociated bar in FIG. 6b can be lowered. Similarly, the heights of thebars in the bar graph can be weighted based on peak noise such that asub-band having a high peak noise can be increased in height by someamount. In any case, weighting can be performed based on thresholds forthe respective statistical values. Weighting can be applied based onindividual statistical values or combinations of statistical values. Theautomatically selected sub-band can be accepted by the operator touchingan Auto-Select button 780 or by touching any sub-band which he or shewishes to choose. The operator can override the automatic selection, forexample, based on which specific transmitters are currently availablefor performing the inground operation. As another basis to presentinformation to the operator, other statistical values can be presented.For example, overbars 781 (a number of which are individuallydesignated) show the peak noise per sub-band. The operator may choose toavoid a sub-band that exhibits a particularly high peak noise level,even if the average noise for that sub-band is relatively low. Forpurposes of over-riding the automatic selection, the operator can toucha Manual Select button 782 and then touch a sub-band which he or shewishes to choose. In another embodiment, display 36 on the locator candisplay a plot, bar graph or any suitable form of display format that isderived from the spectral scan that is shown in FIG. 6a such that theoperator is then allowed to manually select one of the sub-bands, forexample, by touching the sub-band of choice on the display screen. Instill another embodiment, locator 20 can allow the operator to initiallyenter information relating to the transmitters that are available forautomatic selection of a sub-band that is covered by one of thosetransmitters, excluding sub-bands that are not available, in a mannerthat is consistent with the teachings of U.S. Pat. No. 8,729,901 whichis commonly owned with the present application and hereby incorporatedby reference in its entirety. FIG. 6b illustrates sub-bands that are notavailable, based on unavailable transmitters, using dashed lines.Conversely, solid lines indicate sub-bands that are available. In thepresent example, SB-1 and SB-5 through SB-7 are not available. In anembodiment, sub-bands can be excluded based on regulatory constraints.In this way, the portable device itself and the operator are not allowedto make frequency selections that would violate regulations in aparticular jurisdiction. Such frequency restrictions can bepredetermined by the manufacturer on a regional basis. In an embodiment,portable device 20 or some other component of the system such as, forexample, drill rig 80 can be equipped with a GPS receiver that canestablish the location of the inground operation and then look up thelocal frequency requirements.

Still referring to FIG. 6 b, the display screen that is shown can remain“live” at least until the frequency selection process is completed. Thatis, the average noise per sub-band can be monitored and displayed,either alone or weighted by other statistical parameters, in real timefor operator monitoring purposes. In this way, the operator can move thelocator about while observing the average noise in the varioussub-bands. For example, the operator can walk a planned borepath andmonitor the noise along the borepath prior to beginning drilling. Inthis way, a sub-band that is particularly noisy at one or more pointsalong the borepath can be avoided. If the operator so chooses, he or shecan move the locator to a different point, for example, along theborepath and initiate a rescan of the noise across the entire bandwidthby selecting a rescan button 784. As discussed above, the noiseenvironment can be characterized based on reception using one or moreantennas. The operator can change the receiving mode using a button 786.For example, in one receiving mode, the bar chart of FIG. 6b can bepresented based on reception along a single axis such as, for example,the X axis. In another receiving mode, the bar chart can be presentedbased on a vector sum produced from three orthogonal receiving axes.Button 786 can also be used to initiate the rebar mode and/or to togglebetween the rebar and normal modes. In this regard, the appearance ofdisplay 36 can change, as will be seen at an appropriate pointhereinafter. Once the operator changes receiving modes, rescan button784 can initiate a new noise scan and present the noise values based onthe selected receiving mode. The operator can switch between the variousnoise scanning modes at will. In an embodiment, the noise scan thatforms the basis for the display of FIG. 6b can be a high resolutionscan. In conjunction with performing the noise scan, a number ofoptimized, low noise frequencies can be selected automatically based onthe number of symbol frequencies that is needed. For example, sixteensymbol frequencies and a depth frequency can be selected per sub-band.In an embodiment, during the presentation of live noise on the screen ofFIG. 6 b, noise per sub-band can be presented as an average of the noisevalues measured at each of the selected frequencies within eachsub-band. It is noted that selection of rescan button 784 causes a newor updated selection of frequencies within each sub-band. Locator 20 canbe configured to store sets of frequency selections that are associatedwith different measurement positions, for example, in memory 714 of FIG.9 a. Accordingly, the frequency selections are optimized for eachmeasurement position such that different selections can be used atdifferent times during the operation. The term “optimized” is intendedto mean that the selected frequencies are chosen with the intent ofavoiding interference based on one or more statistical parameters suchas, for example, average noise, standard deviation and peak noise. Thefrequency selection sets can be communicated to the transmitter, forexample, above ground using external communication connection 174 ofFIG. 3. An inground transmitter can be commanded in any suitable mannerto switch to a different set of frequency selections during the ingroundoperation. For example, switching between the rebar and normal modes canbe commanded based on a predetermined roll sequence of the drill stringor by transmission of an electromagnetic signal from above ground forreception by transmitter 130 which is, in this case, configured as atransceiver. Some embodiments can use the drill string as an electricalconductor or can include a well-known wire-in-pipe arrangement such thatdata can be transmitted between the inground transmitter/transceiver andthe drill rig. For example, the drill rig can send a command via thedrill string to cause a switch between the rebar and normal modes. Oneembodiment to initially establish operation in either the rebar ornormal mode is based on the orientation of the transmitter at startupwhen the batteries are installed. For example, one end of thetransmitter can be oriented upward to establish the normal mode whileorienting that same end downward establishes the rebar mode.

FIG. 6c illustrates one embodiment of a screen shot showing display 36including a bar graph illustrating noise measurement results foroperation in the rebar mode wherein sub-band SB-4 is highlighted, forexample, using hatching and/or color or in some other suitable manner toindicate that SB-4 has been automatically selected for purposes of datatransmission. At the same time, bars 790, 792 and 794 are associatedwith rebar bands RBL, RBM and RBH, respectively. In this instance, theheight of each column for the rebar bands shows the measured activenoise for the lowest noise frequency, that can be referred to as apotential depth frequency, that was detected by scanning in each rebarband, whereas the columns for SB-2 through SB-4 can indicate an averagevalue for the data symbol frequencies in each sub-band, as discussedabove. Crosshatching or some other suitable visual expedient canindicate an automatic selection or a recommendation of the lowest noisefrequency in the rebar bands which is RBM in the present example,although this is not required. The operator can select, override orchange the selected rebar frequency, for example, by touching the bandcolumn. It should be appreciated that one noise scan of the entirebandwidth can be used to generate the screens of both FIGS. 6b and 6 c,although this is not a requirement. Subsequent, to the noise scan, thedisplay of FIG. 6c can present a live noise indication for each of therebar bands such that, for example, the operator can walk the intendedborepath and monitor the noise for variation therealong. In anotherembodiment, a plurality of depth frequencies can be transmitted. Forexample, the selected frequency for each of the RBL, RBM and RBH rebarbands can be transmitted in a sequence of intervals for above groundreception. It should be appreciated that such a plurality of frequenciescan be selected in the normal mode and transmitted in a like manner.Reception of a plurality of depth frequencies will be discussed at anappropriate point hereinafter.

FIG. 7 is a further enlarged view of sub-band 10 from FIG. 6 a,generally indicated by the reference number 550, and is shown here toillustrate the selection of a depth frequency and sixteen symbolfrequencies S0-S15 within this sub-band for the normal mode. It is notedthat the selection of symbol frequencies for data transmission in therebar mode can be performed in a like manner, based on noise scans ofthe available frequency range for data transmission (sub-bands SB-2through SB-4 in FIG. 5c ), while the rebar depth signal frequency isseparately chosen from rebar band set 424, as illustrated by FIG. 6 c.In FIG. 7, each selected frequency has been designated by an arrow. Thevarious frequencies have been selected, for example, based on theircorrespondence to low noise points in the noise plot. Based on theselection of frequencies, such as S0-S15 either automatically and/ormanually, Applicants submit that system 10 can provide a level of noiseimmunity that has heretofore been unseen with respect to performing aninground operation such as, for example, horizontal directional drillingand related pull-back or back-reaming operations. The rebar modedisclosed herein is submitted to provide heretofore unseen capabilitiesin terms of immunity to passive interference. Related considerations andfurther details will be provided in the context of a discussion ofdevice 20 which receives the depth signal and the data signal and whichalso can assist in the identification of the depth signal frequency andsymbol frequencies to be used by the transmitter.

FIG. 8 is a flow diagram that illustrates an embodiment for theoperation of a transmitter, generally indicated by the reference number600, according to the present disclosure. It is noted that, for purposesof the present discussion, it will be assumed that the depth frequencyas well as the frequencies associated with symbols S0-S15 have alreadybeen selected for each of the normal mode and the rebar mode. Thesefrequency choices can be stored at any suitable location such as, forexample, in depth frequency table 224 and symbol frequency table 228 ofFIG. 3. The method begins at 602 and proceeds to 604 for selection ofeither the normal mode or the rebar mode in any suitable manner. At 606,the depth multiplier is read. At 608, the transmitter looks up the depthfrequency increment, for example, from increment lookup table 370 (FIG.4) as part of the operation of depth channel frequency control 338. Inan embodiment that uses a single depth channel waveform lookup tablesuch as table 326 in FIG. 4, depth channel phase accumulator 330 canalways address that single waveform lookup table in conjunction withretrieving the appropriate multiplier value from a multiplier table(e.g., multiplier table 376 or 410). On the other hand, in an embodimentthat uses a plurality of depth waveform lookup tables, step 608 also canidentify the correct waveform lookup table such that depth channel phaseaccumulator 330 addresses the appropriate depth channel lookup tablewaveform based on frequency. At step 610, depth channel phaseaccumulator 330 begins counting based on the depth frequency increment,thereby causing lookup table 326 and depth channel PWM generator 350 tobegin continuously generating depth channel frequency 386 to emit depthsignal 120 at this frequency, subject to the multiplier value. At 614,CPU 200 reads sensor information via multiplexer 154 to collect sensordata that is to be transmitted. At 618, the CPU assembles the sensordata into a symbol stream which can invoke a packet structure that isyet to be described. The symbol stream is provided as data stream symbolinput 374 to symbol channel frequency control 340 in FIG. 4. At 619, thedepth signal frequency multiplier(s) can be read. At 620, the symbolchannel frequency control can utilize lookup table 326 to identify theappropriate frequency for a current symbol to be transmitted as well asthe multiplier value (e.g., multiplier table 376 or 410). On the otherhand, in an embodiment that uses a plurality of symbol waveform lookuptables, step 620 also can identify the correct waveform lookup tablesuch that symbol channel phase accumulator 334 addresses the appropriatesymbol channel lookup table waveform. It should be appreciated that thetransmission of a given symbol stream can necessitate that step 620switches multiplier values on a symbol-by-symbol basis from one symbolto the next, based on frequency. At 624, the current symbol istransmitted. Step 626 checks for the availability of another symbol totransmit. If a symbol is available, operation returns to 620 such thatthe process repeats for the next symbol. On the other hand, if the nextsymbol is not yet ready, operation can be directed to 630 which respondsto a request to switch from one of the normal and rebar modes to theother one of the normal and rebar modes. The request can be provided inany suitable manner, for example, based on a roll orientation sequence,data transmission via the drill string and wireless communication. Ifthere is no request, operation can return to 610 which continuestransmission of the depth signal. Sensor data is then again read at 614and the process continues therefrom. On the other hand, if there is arequest to switch bands operation returns to step 606 and proceedstherefrom. It should be appreciated that data signal 122 is most oftentransmitted on an essentially continuous basis simultaneously with depthsignal 120.

Having described embodiments of transmitter 130 in detail above,attention is now directed to FIGS. 9a-9c in conjunction with FIG. 1 forpurposes of describing additional details with respect to device 20which may be referred to interchangeably as a locator or receiver. FIG.9a is a block diagram that illustrates device 20. The latter includes abattery 700 that feeds a power supply 704 which supplies appropriateelectrical power to all of the components of the device, indicated asV+. Electronics section 32 includes a processor 710 that is interfacedwith a memory 714. A telemetry section 720 is controlled by theprocessor and coupled to antenna 40 for bidirectional communication viasignal 44. In some embodiments, the telemetry link can be unidirectionalfrom device 20 to the drill rig, in which case transceiver 102 need onlyinclude receiver functionality. An external communication arrangement722 provides for external communication with a transmitter usingexternal communication connection 174 (FIG. 3) of the transmitter. Asdiscussed above, such communication is not required to be transmittedthrough the ground but rather can be performed while the transmitter isabove ground, for example, in a position adjacent to device 20. Theexternal communication can be implemented in any suitable mannerincluding but not limited to IrDA, NFC, Wi-Fi, Zigbee or Bluetooth. Awide-band dual mode front end 730 is configured for receiving depthsignal 120 and data signal 122 using X, Y and Z antennas which make upantenna cluster 26 for measuring three orthogonal components of thesubject signals as well as for performing noise measurements along theseaxes, as is yet to be described. Additional details with respect to anembodiment of the antenna cluster will be provided at an appropriatepoint hereinafter. Each of the X, Y and Z antennas is interfaced to alow noise amplifier (LNA) 734 a, 734 b and 734 c, respectively, each ofwhich can be identically configured. The amplified output of each LNA issupplied to a respective one of switchable filter sections 738 a, 738 band 738 c, each of which can be configured identically and which may bereferred to collectively as filter sections 738. Each filter sectionincludes two bandpass filters 740 a and 740 b exhibiting a low frequencyroll-off or corner and a high frequency roll-off or corner. Filters 740a may be referred to as normal mode filters while filters 740 b may bereferred to as rebar mode filters. While filters 740 a and 740 b areillustrated as individual functional blocks, it should be appreciatedthat the filters can be implemented in any suitable manner. By way ofnon-limiting example, each filter can be implemented as a series of RChigh-pass and low-pass filters that are distributed throughout thesignal chain. The normalized frequency response of normal mode filter740 a is diagrammatically illustrated by a plot 742 in FIG. 9 b. In anembodiment of the normal mode filter, two high-pass filters can each beset at a low corner frequency of about 4 KHz and four low pass filterscan be set at a high corner frequency of about 90 KHz. This embodimentyields a relatively flat frequency response with a lower cornerfrequency 742 a at about 10 KHz and a high corner frequency at about 50KHz. The roll-off below 10 KHz is approximately 40 dB of attenuation perdecade and the roll-off above 50 KHz is approximately 80 dB ofattenuation per decade. FIG. 9c diagrammatically illustrates anembodiment of the normalized frequency response of rebar filter 740 cindicated by the reference number 744. High corner frequency 743 b canbe unchanged, as illustrated, although this is not required. On theother hand, low corner frequency 745 can be moved downward, for example,to about 180 Hz at a rolloff of about 3 dB. In this way, there is littlesignal attenuation at 300 Hz. It should be appreciated that the low endresponse of filters 740 a and 740 b can be established in considerationof the fundamental and low order power line harmonics, which can be verystrong particularly in the instance of the rebar mode filter. A filterswitching line 747 is shown as a dotted line that extends from CPU 710to each of the filters in filter sections 738. Accordingly, CPU 710 cancontrol filter selection in the normal and rebar modes. Amplifiers 750a-750 c can follow each respective one of filters 738 a-738 c withsufficient gain for purposes of driving each of analog-to-digitalconverters A/D 754 a-754 c. Each A/D 754 provides an output to CPU 710.In an embodiment, device 20 can be configured to receive the symbolstream in a way that suppresses powerline harmonic frequencies sincethere is effectively no energy present in the symbol stream at thepowerline harmonics. For example, the received signal can be processedsuch that the receiver response matches the symbol spectra asillustrated by plot 553 of FIG. 7. In particular, the spectral responseof the receiver can be matched to the spectral characteristics of thetransmitter by integrating the received symbol stream over a time periodthat corresponds to the time duration or period of each symbol. In thisway, the receiver frequency response matches the response of thetransmitter with respect to exhibiting null reception points at thepowerline harmonic frequencies. Accordingly, energy at the harmonicfrequencies is suppressed or ignored by the receiver while sweeping upthe spectral energy that is associated with the symbol. The receiver canemploy any suitable demodulation process that provides periodic nullsincluding but not limited to a Discreet Fourier Transform (DFT).

Referring to FIGS. 9b and 9 c, it is worthwhile at this juncture tocompare the response 742 of the normal mode filter and rebar mode filter744 in view of the spectral plot of FIG. 6a as well as in view of thediscussions above with regard to high active noise below what Applicantssubmit to be an ultralow depth frequency range such as, for example,below 1 KHz. In this regard, low frequency corner 743 a of the normalmode filter is positioned at approximately 4 KHz whereas low frequencycorner 745 of the rebar mode filter is positioned no higher thanapproximately 180 Hz. While lowering the low frequency corner of therebar filter allows for detection of the depth signal at very low depthsignal frequencies at least down to 300 Hz or down to an appropriatepowerline frequency, it should be appreciated that a serious technicalchallenge is simultaneously introduced. In particular and as seen inFIG. 6 a, the rebar mode filter admits a large amount of noise between180 Hz (or even lower) and 4 KHz that is normally rejected by the normalmode filter. This noise can contain, for example, very strong powerlineharmonics. It is submitted that the admission of all this noise would becounter intuitive to one of ordinary skill in the art and would beperceived as likely to render the portable device as inoperable forpurposes of receiving such low depth signal frequencies as well asintroducing additional difficulty with regard to receiving higherfrequencies. Applicants, however, recognize that the combined teachingsherein provide for admitting this additional noise to enable thereception of very low frequency depth signals while, at the same time,providing the capability to position the depth signal at a precisefrequency in the rebar bands that can be free of noise, at least to areasonable approximation. In this way, the depth signal can be detectedwith some assurance of immunity to the generally high ambient noiselevels below 1 KHz or some other suitable limit such as, for example,1.5 KHz. Applicants are unaware of any suggestion of these combinedfeatures in the prior art.

Referring to FIG. 9a and having described an embodiment of locator 20 indetail above, it should be appreciated that the locator can beconfigured for performing noise measurements and analysis for purposesof selecting a transmitter for transmission of the depth signal and thedata signal as well as establishing the frequencies to be associatedwith each of these signals. Of course, band selection may not berequired when a wideband transmitter is used. Noise measurements can bedetermined based on each orthogonal axis of antenna 26 (X, Y and Zantennas, as shown in FIG. 9a ). These individual noise components canbe used to establish a three dimensional noise value, for example, basedon a vector sum of the three antenna components. The vector sum can beuseful since the noise reading at a given point will essentially beinvariant with changes in the orientation of the locator. On the otherhand, displaying the noise reading obtained from a single axis willgenerally exhibit variation at a given point as the orientation of thelocator is changed. By allowing for monitoring noise along a singleantenna axis such as for example the X axis, an operator can identifywhich particular axis along the bore path may be problematic in terms ofinterference. Noise values can be determined in any suitable manner suchas, for example, based on a Fast Fourier Transform (FFT). In anembodiment, a noise scan can be produced from each axis for comparativepurposes. For example, an axis that exhibits relatively higher noisethan the other axes can be handled differently for purposes of datarecovery.

Attention is now directed to FIG. 10a which is an expanded view ofsub-band 6 from FIG. 6 a, generally indicated by the reference number800. For purposes of the present discussion, it will be assumed thatSB-6 is available and has been selected either automatically or by theoperator for use during the inground operation. It is noted thatsymbol/data frequency selection can proceed for any sub-band inaccordance with these descriptions. Operation in the normal mode isassumed, at least initially. Having selected a sub-band, the frequenciesfor depth signal 120 and data signal 122 can be established. In anembodiment, the frequencies can be predetermined, for example, by themanufacturer or based on a previous noise scan, as described above. Inanother embodiment, display 36 can be used to represent the spectralplot of FIG. 10 a, in any suitable form, to an operator of the locatorsuch that the operator can make frequency selections. FIG. 10billustrates one embodiment of a screen shot which shows display 36illustrating SB-6, still assuming the normal mode. It should beappreciated that the locator can provide a zoom function on display 36that uses Zoom In button 802 and Zoom Out button 804 such that theoperator can expand the horizontal extents of the spectral display toprovide for detailed frequency selection. Generally, the operator canselect frequencies that correspond to low noise points on the displayedspectrum. The selections can be rounded to reflect the frequencyresolution of the transmitter that is to be used. As discussed above,embodiments of transmitters according to the present disclosure can havea frequency resolution of 5 Hz, by way of non-limiting example.Twenty-one low noise points are identified on FIG. 10a indicated asupticks (a)-(u). In an embodiment using one depth frequency for depthsignal 120 and 16 symbol frequencies, seventeen of these 21 frequenciescan be utilized. As described above, the depth frequency can be locatedat any position within the sub-band, intermingled with the symbolfrequencies, at either end of the sub-band or even in a differentsub-band. As one example, the depth frequency can be selected as thelowest noise point among the identified frequencies, which is frequency(j) in the present example. In still another embodiment, the frequenciescan be automatically picked or re-picked by locator 20, for example,responsive to the operator selecting an “Auto-Pick” button 806 ondisplay 36. In one embodiment, processor 710 can examine the spectrum ofFIG. 6a to identify the lowest noise points until a suitable number ofsymbol frequencies is available. In other embodiments, the processor canperform the selection process based on any suitable method. For example,the lowest noise frequencies can be selected in conjunction withmaintaining a minimum separation between adjacent frequencies. Withregard to the rebar mode, the frequency selection can include selectionsfor data symbols that can be essentially identical in terms of thedisplay of FIG. 10b with an additional selection of the depth frequency,for example, in rebar band set 424 which can be based on a display ofrebar bands 424, as seen in FIG. 6c or a display of noise scan resultsfor the rebar band set or some portion thereof which can have anappearance that is similar to or based on FIG. 10 b. In this way, theoperator is provided with a great deal of flexibility for purposes ofselecting the depth frequency in the rebar mode.

Still referring to FIG. 10 b, a frequency can be added, for example, bytouching an Add Frequency button 808 and then touching the spectralplot. A frequency can be deleted, for example, by touching a DeleteFrequency button 810 and then touching the frequency to be deleted. Afrequency can be moved, for example, by touching a Move Frequency button812 and then touching and dragging the frequency to be moved. Theselected sub-band can be changed by touching a Change Sub-Band button814. As will be further described immediately hereinafter, frequencyselection is not limited to identification of low noise points but alsocan consider high noise points or areas of the spectral scan.

FIG. 11 is a further expanded view of the spectral region of FIG. 10afrom 24 KHz to 25 KHz, generally indicated by the reference number 820and shown here for purposes of describing further details with respectto frequency selection. In addition to identifying low noise points, asdescribed with regard to FIG. 10 a, processor 710 can apply what can bereferred to as a “keep-out region”. The later will exclude anyidentified low noise frequency having a noise peak within a selectedfrequency window 822 that is centered on that low noise frequency. Thenoise can be identified, for example, based on a magnitude that exceedsa threshold 824 based on the average noise value for the sub-band and/orthe noise value associated with the nearby low noise point. In anembodiment, the frequency window can be approximately 60 Hz (±Hz) inwidth and the threshold can be 10 dB or more above the associated lownoise point. Based on the use of such a frequency window, frequencies(b) and (e) can be excluded due to the proximity of peaks 826 and 830,respectively. In the event that more frequencies are needed, processor710 can re-examine the spectrum of FIG. 11 for purposes of identifying anew set of frequency candidates. In the rebar band set, a rebarfrequency window can be of a different width than the frequency windowin the normal mode. For example, the rebar frequency window can be morenarrow since noise scanning can be performed at a small increment suchas, for example, 5 Hz.

FIG. 12a is a flow diagram that illustrates an embodiment of a method,generally indicated by the reference number 900, for the operation oflocator 20 in performing spectral scanning and frequency assignment forsubsequent operation in the normal mode in accordance with the presentdisclosure. The method begins at 904 and proceeds to 908 which performsa scan of the full frequency spectrum, for example, from 0 Hz to 45 KHzfor the present embodiment, although any suitable range can be used forthis scan. The scan can be a high-resolution scan, for example,utilizing a resolution of 5 Hz, as discussed above. In anotherembodiment, an initial, lower resolution scan can be utilized such thatthe resolution is just sufficient to establish an average noise valuefor each sub-band. If the sub-band selection process relies on a lowerresolution spectral scan, a high resolution spectral scan cansubsequently be performed as part of the frequency selection procedure,described below. When a wideband transmitter will be used for theinground operation, a single high resolution scan can be employed forfrequency selection purposes. At 910, the average noise value persub-band is determined. At 914, a sub-band can be recommended based onthe average noise values. Generally, the sub-band having the lowestaverage noise value can be recommended, although other embodiments canutilize different recommendation protocols and/or automated selection.For example, the sub-band having the lowest noise peak value can berecommended. By way of another example, as discussed above, more thanone sub-band can be recommended. At 918, user input can be requested ondisplay 36 wherein the user can accept the recommended sub-band orchoose a different sub-band. For example, the user may choose adifferent sub-band based on an awareness of transmitters that areavailable for performing the inground operation. As discussed above,this information can serve as an initial input such that method 900excludes sub-bands that are not covered by the currently availabletransmitter(s). Once the sub-band has been selected, the method proceedsto 920 which determines the sub-band frequencies. In an embodiment, thesub-band frequencies can be predetermined and stored in memory 714 ofthe locator or in memory 218 of the transmitter. In another embodiment,the sub-band frequencies can be determined by the operator on-the-fly bypresenting the sub-band on display 36, as discussed above. In anotherembodiment, the sub-band frequencies can be determined automatically inaccordance with the discussions relating to FIGS. 10a-10c and 11 with orwithout the application of a keep-out window, as applied by step 924. Inthe instance of the use of a wideband transmitter for performing theinground operation, steps 910 and 914 are not required since the entiretransmission bandwidth can be available for frequency selection withoutthe need to confine the frequencies to any particular sub-band(s) andstep 920 can allocate frequencies across the entire transmissionbandwidth. Accordingly, transmission frequencies can be selectedautomatically across the entire available bandwidth and/or customized bythe user based on a high resolution noise scan without the need forfrequency assignment limitations based on sub-bands. It should beappreciated that an embodiment of a wideband transmitter can beconfigured to operate, for example, based on operator preference, usingsub-bands in the same manner as sub-band limited transmitters whereinfrequency assignment can be confined to one or more sub-bands, althoughthis is not required. At 928, a determination is made as to whether asufficient number of frequencies have been identified. If not, operationreturns to 920 for identification of additional frequencies. If asufficient number of frequencies have been identified, operationproceeds to 930 which recommends frequencies for depth signal 120 anddata signal 122. This latter step may be optional in a fully automatedembodiment. At 934, information can be presented on display 36 forpurposes of gathering user input, for example, approving the frequencyselections or changing the frequency selections. For instance, the usermay prefer to move the depth frequency to a different location withinthe sub-band or to an altogether different sub-band. Of course, in awideband transmitter embodiment, no restrictions need be imposed withrespect to limiting frequency selection to a particular band and/orsub-band. At 938, the frequency selections can be transferred totransmitter 130 using external communication arrangement 722 of thelocator and external communication link 174 (FIG. 3) of the transmitter.Normal mode operation can then be entered during the ingroundoperational procedure at 940.

In an embodiment of method 900, the number of frequencies that isselected can be based on the noise environment. For example, if a noisescan, whether sub-band limited or not, shows a low noise environment,relatively more frequencies can be selected. In this case, 32 or moresymbol frequencies can be used instead of 16 symbol frequencies. If thenoise scan shows a high noise environment, relatively fewer symbolfrequencies can be used such as, for example, 4 or 8 symbol frequenciesinstead of 16 frequencies. Generally, the use of relatively fewerfrequencies can aid in avoiding variable noise sources in a highinterference environment. On the other hand, using a higher number ofsymbol frequencies can increase data throughput.

FIG. 12b is a flow diagram that illustrates an embodiment of a method,generally indicated by the reference number 944, for the operation oflocator 20 in performing spectral scanning and frequency assignment forsubsequent operation in the rebar mode in accordance with the presentdisclosure. The method begins at 946 and proceeds to 948 which canperform a scan of the full frequency spectrum, for example, from 0 Hz to45 KHz for the present embodiment, although any suitable range can beused for this scan. For example, based on FIG. 5 c, the scan can be fromapproximately 0 Hz to 18 KHz. The scan can be a high-resolution scan,for example, utilizing a resolution of 5 Hz, particularly for rebar bandset 424. Lower resolutions can be used for purposes of selecting symbolfrequencies, as described above. At 950, the average noise value persub-band can be determined for symbol frequency sub-bands and thescanned frequencies for potential use as the depth signal frequency canbe analyzed. A sub-band for data symbols can be recommended and/orselected based on any suitable statistical value such as, for example,average noise values, minimum noise values, peak noise values and/orstandard deviation values in a manner that is consistent with thedescriptions above in conjunction with recommending one or more depthsignal frequencies. At 954, user input can be requested on display 36wherein the user can accept the recommended sub-band or choose adifferent sub-band for the data symbols. The user can also select,modify or confirm one or more potential depth signal frequencies. It isnoted that user input can be optional in an automated embodiment thatperforms frequency selection for the user. Once the sub-band and depthsignal frequencies have been selected, the method proceeds to 956 whichsets the sub-band frequencies and the depth frequency. In an embodiment,the sub-band frequencies and depth frequencies can be predetermined andstored in memory 714 of the locator or in memory 218 of the transmitter.In another embodiment, the sub-band and/or depth frequencies can bedetermined by the operator on-the-fly by presenting the sub-band and/orthe rebar bands on display 36, as discussed above. In anotherembodiment, the sub-band and/or depth frequencies can be determined andapplied automatically in accordance with the discussions relating toFIGS. 10a-10c and 11 with or without the application of a keep-outwindow, as applied at 958 and discussed above.

It is noted that determinations and indications based on sub-bands instep 950 are not required, as discussed above, since there is norequirement to confine the frequencies to any particular sub-band(s) andstep 950 can allocate frequencies across the entire transmissionbandwidth. Accordingly, transmission frequencies can be selectedautomatically across the entire available bandwidth and/or customized bythe user based on a high resolution noise scan. It should be appreciatedthat an embodiment of a wideband transmitter can be configured tooperate, for example, based on operator preference, using sub-bands inthe same manner as sub-band limited transmitters wherein frequencyassignment can be confined to one or more sub-bands, although this isnot required. At 960, a determination is made as to whether a sufficientnumber of frequencies have been identified. If not, operation returns to950 for identification of additional frequencies. If a sufficient numberof frequencies have been identified, operation proceeds to 962 whichrecommends frequencies for depth signal 120 and data signal 122. Thislatter step may be optional in a fully automated embodiment. At 964,information can be presented on display 36 for purposes of gatheringuser input, for example, approving the frequency selections or changingthe frequency selections. For instance, the user may prefer to move thedepth frequency to a different location within a rebar band or to analtogether different rebar band. Of course, in a wideband transmitterembodiment, no restrictions need be imposed with respect to limitingsymbol frequency selection to a particular band and/or sub-band. Onceagain, there is no requirement for user input in an automatedembodiment. At 966, the frequency selections can be transferred totransmitter 130 using external communication arrangement 722 of thelocator and external communication link 174 (FIG. 3) of the transmitter.The rebar mode can then be entered at 968 during the ingroundoperational procedure.

Attention is now directed to FIG. 12c in conjunction with FIG. 12 b.FIG. 12c is a flow diagram that illustrates an embodiment of a methodfor step 950 of FIG. 12 b, generally indicated by the reference number970, for purposes of analyzing the scanned frequencies in the depthband(s). The method begins at start 972 and proceeds to 974 whichcompares the noise values obtained from the noise scan for each of rebarbands RBL, RBM and RBH of FIG. 5c to identify, at 976, at least thefrequency in each of these rebar bands that exhibits the lowest noisevalue. In this embodiment, the result is a set of three low noisepotential depth signal frequencies. At 978, the set of low noisefrequencies can be displayed to the user, for example, based on thescreen of FIG. 6c , and/or an auto-select of the lowest noise frequencyfrom the set can be performed. At 980, operation can return to step 950of FIG. 12 b.

Turning now to FIG. 12d in conjunction with FIG. 12 b, the former is aflow diagram that illustrates another embodiment of a method for step950 of FIG. 12 b, generally indicated by the reference number 982, forpurposes of analyzing the scanned frequencies in the depth band(s). Themethod begins at start 983 and proceeds to 984 which compares pluralityof noise values obtained from the noise scan for the overall rebar noiseband or range to a threshold noise value. The latter can at leastinitially be a low value for reasons that will be evident in view of thecontinuing discussion. At 985, low noise frequencies are identified thatdo not exceed the threshold. Step 986 tests whether any frequencies wereidentified, as a result of the low value for the noise threshold. Iffrequencies were identified, operation branches to 987 which candisplay, auto-select and/or confirm the depth signal frequency withcooperation from the operator as desired. At 988 operation returns tostep 950 of FIG. 12 b. If, on the other hand, no frequencies wereidentified at 986 since all the scanned frequencies exhibited noiselevels above the threshold noise level, operation branches to 990, whichcan query the operator to trigger a re-scan of the noise at least in therebar band. It is noted that decision 986 is not a requirement. In thisregard, any suitable response to excess or severe active interferencecan be provided. For example in another embodiment, an indication can beissued to the operator that there is severe noise present in the rebarband. In the present embodiment, if a rescan is selected, operationproceeds to re-scan at 991 with a subsequent return to 985. If the userdoes not request a re-scan at 990, operation can proceed to 992 whichqueries whether the user would like to raise the noise threshold. If so,the noise threshold can be increased, for example, by a predeterminedincremental amount or the user can specify the increase. Operation canthen return to 991 and proceed therefrom. If the user does not choose toincrease the noise threshold at 992, operation can move to 993 whichqueries whether the user would like to leave the rebar mode and enterthe normal mode, assuming that there is too much local active noise inthe ambient operational environment. If the user selects to return tothe normal mode, the normal mode is entered at 994 which can invoke, forexample, method 900 of FIG. 12 a. On the other hand, if the user doesnot choose to enter the normal mode at 993, operation proceeds to 995which is not required but involves selecting the depth signal frequencybased on meeting a different set of noise specifications. Operation thenproceeds to 987. One embodiment for step 995 can be seen in FIG. 12 e,as described immediately hereinafter, although the procedure of FIG. 12ecan be entered independent of method 982 of FIG. 12 d.

Reference is now made to FIG. 12 e, which illustrates another embodimentof a method for step 950 of FIG. 12 b, generally indicated by thereference number 1000 or which can serve as step 995 in FIG. 12d forpurposes of analyzing the scanned frequencies in the depth band(s). Itis noted that method 1000 can serve as a primary embodiment foridentifying a depth signal frequency and/or as a further vettingprocedure (step 995 of FIG. 12d ) to ensure that an identified frequencyis as low as practical, as will be made evident in the descriptions thatfollow. In this regard, it is noted that passive interference immunityis generally enhanced with decreasing frequency such that it can bebeneficial to employ the lowest practical frequency. The method beginsat start 1004 and proceeds to 1006 which finds the lowest noisefrequency, f(low), in the rebar band based on the noise scan, forexample, at a 5 Hz increment. At 1008, although this is not required, anoffset from the lower limit of the rebar band can be determined. At1010, the offset can be compared to a threshold offset which wouldindicate that f(low) is already sufficiently close to the lower limit ofthe rebar band. If the offset is determined to be acceptable, theidentified low noise frequency can be indicated and/or auto-selected at1014. At 1016, the procedure can return, for example, to an overallfrequency selection procedure that also selects symbol frequencies suchas, for example, 950 of FIG. 12 b. On the other hand, if 1010 determinesthat the offset is too high, operation can route to 1020 which sets anindex value, n, that counts the frequency increments for the noise scanof the rebar band starting at the lower limit (n=1) of the rebar bandand ending at n(low) which is the value of n at f(low). At 1024, if thenoise value for the frequency identified by the current value of n iswithin limits, operation can proceed to 1014. It is noted thatacceptable limits can be defined in any suitable manner such as, forexample, no more than some specified amount above the measured noise atf(low). If the noise is not within limits, operation is routed to 1026which increments n. Step 1028 then tests whether the current value for nis equal to n(low). If so, step 1030 can display and/or auto-selectf(low) as the depth signal frequency. Thereafter, operation can returnto step 950 of FIG. 12 b. If the value of n at step 1028 is less thann(low), operation returns to 1024 for comparison of the noise value atthe indicated frequency to acceptable limits, as discussed above. Inthis way, a loop is formed which tests every incremental scanned depthfrequency, up to f(low), such that an acceptable frequency can be foundthat is lower than the actual lowest noise frequency within the rebarband.

FIG. 13 is a flow diagram illustrating an embodiment of a method foroperation of locator 20 during an inground operation, generallyindicated by the reference number 1100. The method begins at 1104 andproceeds simultaneously along a depth determination branch 1110 and adata recovery branch 1112. Depth branch 1112 receives depth signal 120at 1120 and then determines the depth of the transmitter at 1124.Because the depth signal is transmitted on a dedicated frequency, thedepth signal is receivable on an essentially continuous basis throughoutthe inground operation. Accordingly, steps 1120 and 1124 repeat in aloop fashion throughout the normal operation mode of the locator. Asdescribed above, step 1124 can utilize the depth signal to determine thedepth of the transmitter based on the dipole equations. In anembodiment, part of the depth determination can include compensation forthe distance of the locator above the surface of the ground. In anembodiment for receiving a plurality of depth frequencies such as, forexample, from the RBL, RBM and RBH bands (see FIG. 6c ), it is notedthat an average signal strength of the signals can be used for purposesof depth determination. Data recovery branch 1112 begins at 1130 withreception of data signal 122 in the form of a symbol stream that can bemade up of multi-bit symbols. At 1134, the symbol stream can betemporarily stored for decoding, for example, in memory 714 (FIG. 9a ).At 1138, processor 710 decodes the symbol stream. In this regard, one ofthe symbols can be used as a synchronization symbol that can identifythe start of a packet structure. In an embodiment that uses a 4 bitsymbol (i.e., 16 symbol frequencies), a seventeenth symbol frequency canbe added for purposes of representing a synchronization symbol in thesymbol stream. One suitable packet structure, by way of non-limitingexample, can be represented by a series of 4-bit variables as S, P1, R1,P2, R2, BT1, BT2, R3 wherein S has a fixed value that corresponds to thesync symbol, P1 is a variable representing the first four bits (0-3) ofa pitch value, R1 is a first roll variable characterizing the rollorientation, P2 is bits 4-7 of the pitch value, BT1 is a first four bits(0-3) of battery and temperature data, BT2 is bits 4-7 of battery andtemperature data, and R3 is a third roll variable. In this regard, itshould be appreciated that the pitch value is accumulated based on twodifferent variables corresponding to two symbols in the symbol streamthat are separated by another symbol. That is, the four bits of P2 canbe appended to the four bits of P1 to represent a complete pitch value.Still further bits can be appended based on another pitch variable, ifdesired. Similarly, 8 bits of battery and temperature data can beassembled based on two successive variables BT1 and BT2. At 1140, a datastream can be reassembled based on the decoded symbol stream toreconstruct the original data that was the basis for the symbol streamin transmitter 130. At 1144, processor 710 recovers parameters from thedata stream. These parameters can represent orientation parameters suchas pitch and roll, temperature, pressure, battery voltage and current,and any other data that is of interest. At 1148, processor 710 respondsto the recovered parameters in any suitable manner such as, for example,by driving display 36 to indicate pitch and roll, battery status,temperature and pressure and/or as inputs for other processes such as,for example, providing warnings when thresholds relating to temperatureand pressure have been violated. Operation then returns to step 1130.

At this juncture, it is appropriate to consider further details withregard to the operation of transmitter 130 during an inground operation.Embodiments of the transmitter can flexibly allocate transmission powerbetween the various frequencies that are transmitted such as, forexample, between the frequencies that are shown in Tables 3 and 4. Inone embodiment, each frequency can be allocated an equal amount oftransmission power. In another embodiment, transmission power can beallocated non-uniformly among the frequencies. For example, one or morefrequencies can be assigned a higher transmission power than anothergroup of frequencies. In still another embodiment, each frequency can beassigned a different transmission power. Such power allocation can beperformed in any suitable manner. For example, portable device 20 ofFIGS. 1 and 9 can be configured to monitor the average signal strengthassociated with each frequency as each frequency is received duringnormal operation. Transmission power can then be reallocated on-the-flyamong the frequencies based on a running average signal strength. Forexample, a sudden decrease in signal strength of a given frequency canbe attributable to interference such that additional power can beallocated to that frequency. In some embodiments, low noise frequenciescan be allocated relatively lower transmission powers while higher noisefrequencies can be allocated relatively higher transmission powers. Thereallocated transmission power values can be transferred to transmitter130 in any suitable manner. For example, portable device 20 can transmitthe reallocated power scheme to drill rig 80 via telemetry signal 44.The drill rig can then transfer the new power scheme to transmitter 130via the drill string by using the latter as an electrical conductor. Inanother embodiment, portable device 20 can be configured with anadditional antenna 712 (FIG. 1) such as a dipole antenna fortransmitting a signal for direct reception by transmitter 130.Modulation of this signal can be decoded by transmitter 130 to recoverthe new power scheme.

Referring again to FIG. 10 b, power allocation among selectedfrequencies can also be performed during the frequency selection processor mode, prior to entering normal mode or rebar mode. For example, powerallocation can be based on a noise value that is associated with eachselected frequency, as shown in FIGS. 10a and 10 b. Although the lownoise frequencies identified in FIGS. 10a and 10d appear to exhibitrelatively equal noise values for illustrative purposes, this may notnecessarily be the case. If there is significant variation among thenoise values for the lowest noise frequencies that are identified,transmission power can be allocated in a higher proportion to thosefrequencies exhibiting relatively higher noise values. Conversely,transmission power allocated to a very low noise frequency can berelatively low to allow for additional power allocation to one or moreother frequencies. Transmission power can also be allocated in a mannerthat is consistent with the application of a keep-out window, asdescribed above. For example, if a particular frequency is selected suchthat a powerline harmonic or other noise anomaly falls within a keep-outwidow for that particular frequency, additional power can be allocatedto the particular frequency. It should be appreciated that in anyembodiment that uses allocated transmission power that can vary fromfrequency to frequency, such allocation can be performed based onoverall power consumption, particularly when transmitter 130 is batterypowered. In this way, the overall power consumption can be reduced or atarget overall power consumption can be maintained.

Applicants recognize that transmission of the depth signal at lowfrequencies in the rebar mode, as disclosed herein, in relation to datasymbol frequencies that are far greater, raises a particular concern. Inthis regard, as the depth signal frequency decreases relative to a givendata signal frequency, even with no change in active interference and inthe absence of any countermeasures, the range at which the depth signalcan be received has been observed by Applicants to decrease relative tothe range at which the data signal can be received. This can be veryproblematic during an operational procedure since locator 20 can losereception of depth signal 120 well before loss of data signal 122. Inorder to ensure full system capability, it is necessary for the locatorto receive both the depth signal and the data signal. While notintending to be bound by theory, Applicants believe that the lowfrequency reduction or decay in range is attributable to reducedcoupling efficiency between the transmit and receive antennas. At thesame time, the antenna behaves primarily as an inductor such that theimpedance of the antenna decreases as frequency decreases. For a fixeddrive voltage, current increases in an at least generally linear manner.Applicants have observed that the decay in the signal strength andthereby range of the depth signal is complex in nature. In embodiments,appropriate transmit powers, in order to compensate for the decay, canbe characterized in any suitable manner such as, for example,empirically. A multiplier table can compensate for this coupling decayeffect or a function can be determined, for example, based on curvefitting for use by the processor of the transmitter to set transmitpower based on frequency. In order to maintain an at least approximatelyequal reception range for the depth signal and the data signal, one canprovide, for example, for adjusting the depth signal up to at leastapproximately 100 times more power than the data signal. This adjustmentcan accommodate adjusting the power allocated to the data signaldownward in order to accommodate rebalancing or shifting power to thedepth signal while not violating the maximum power threshold oradjusting the power allocated to the depth signal downward whileallocating additional power to the data signal for the same reason. Asanother example, for a depth signal frequency at 330 Hz and a datasignal frequency at 18 KHz, the depth signal frequency can be providedwith about 5 times the power of the data signal frequency. In view ofthese recognitions, Applicants bring to light an advanced system inwhich a processor of the transmitter is configured for generating thedepth drive input at the depth signal frequency and for generating thedata drive input, characterizing the sensor data, in a way that controlsa depth signal transmit power in relation to a data signal transmitpower such that a first reception range of the depth signal at leastapproximately matches a second reception range of the data signal eventhough the depth signal frequency is far lower (e.g., 1 KHz or less)than the data signal frequency. Stated in another way, compensation isapplied by balancing transmit power between the depth signal and thedata signal such that the reception range for both signals is at leastapproximately equal. At the same time, this balancing can accommodate afurther requirement to manage total power consumption of the transmittersuch that, for example, a specified maximum power consumption is notexceeded. In an embodiment, multiplier table 410 of Table 3 can beconfigured, at least approximately, based on the aforementioned decaycharacteristic, although this is not required. In Table 3, it should beapparent that the transmit power, based on the multiplier value.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form or formsdisclosed, and other modifications and variations may be possible inlight of the above teachings. Accordingly, those of skill in the artwill recognize certain modifications, permutations, additions andsub-combinations of the embodiments described above.

What is claimed is:
 1. A portable device as part of a system in which atransmitter is configured to move through the ground in a region duringan operational procedure while transmitting a depth signal, saidportable device comprising: an antenna having an output; a receiverconfigured to scan a plurality of rebar frequency bands in a frequencyrange that is below 4.5 KHz to measure active electromagnetic noisewithin each rebar frequency band based on the antenna output andidentify a lowest active noise frequency within each one of the rebarfrequency bands; and a display for displaying the measured level ofactive electromagnetic noise associated with each rebar frequency bandfor use in selecting the depth frequency to provide at least someimmunity to passive interference.
 2. The portable device of claim 1wherein a lower limit of the frequency range is 330 Hz.
 3. The portabledevice of claim 1 wherein the portable device is configured to recommenda potential depth frequency to a user selected from the identifiedlowest active noise frequencies.
 4. The portable device of claim 1wherein the portable device is configured to automatically select apotential depth frequency as the depth frequency from the identifiedlowest active noise frequencies.
 5. The portable device of claim 4wherein said portable device is configured for a user override of thepotential depth frequency in serving as the depth frequency.
 6. Theportable device of claim 4 wherein the portable device is configured atleast to request a confirmation of the potential depth frequency fromthe user.
 7. The portable device of claim 1 configured to enter a livenoise scan mode for scanning the electromagnetic noise along an intendedpath at least at a potential depth frequency that is selected from theidentified lowest active noise frequencies to indicate a live noisevalue to the operator for the potential depth frequency.